#include <fstream>
#include <iomanip>
#include <ios>

int main()
{
   std::ifstream input("in.txt");
   std::ofstream output("out.txt");

   double x_0, y_0;
   input >> x_0 >> y_0;

   double x_1 = x_0, y_1 = y_0;
   
   double x_2, y_2;
   
   double s = 0;

   while (input >> x_2 >> y_2)
   {
	   s = s + (x_1 + x_2) * (y_1 - y_2);
	   x_1 = x_2;
	   y_1 = y_2;
   }
   s = 0.5 * abs(s + (x_1 + x_0) * (y_1 - y_0));

   output << std::fixed << std::setprecision(10) << s << std::endl;

   return 0;
}